1. Field of the Invention
The present invention relates to non-impact printer apparatus for recording, and more specifically, to circuitry thereon for controlling signals flowing to a printhead forming part of the printer apparatus to control current uniformity to the recording elements.
2. Description of the Prior Art
In U.S. Pat. No. 4,831,395, an L.E.D. (light emitting diode) printhead is described wherein control of current to the LED's during recording is provided by a current mirror circuit. This circuit features an adjustable resistor and voltage supply to allow the respective currents to the LED's to be controlled so that adjustments may be provided to have the outputs of the LED's be more uniform. Where groups of LED's are driven from respective driver chips, each driver chip is separately adjustable so that LED's driven from this driver chip receive sufficient current so that their respective intensities are similar to that of LED's driven by other driver chips on this printhead. Another aspect of adjustability requires the LED's to put out sufficient light to expose the recording medium such as a photoconductor.
In U.S. Pat. No. 4,885,597, an LED printhead is described wherein compensation for age and temperature of the LED's is accommodated to ensure that the intensity from the LED's is consistent over time. In this printhead, current to the LED's is regulated using digitally addressable current mirrors associated with each driver chip. The multi-bit digital current regulation signals used must be communicated to the driver chips and adjustments in intensities of the LED's made accordingly. A problem with providing adjustability to current is that temperature is a localized matter with temperature being different from chip to chip and varying with time in accordance with demand for printing. Providing a few thermistors upon a printhead may not provide sufficient information relative to temperature that can be used to very accurately control uniformity. While adding more thermistors can increase accuracy doing so entails added expense both in materials and manufacturing cost for the printhead and still does not provide information relative to temperature inside the driver chip.
It is therefore an object of the invention to overcome this problem and to provide an improved printer apparatus with improved means for measuring localized temperature of the printhead and for providing for more prompt and fine-tuned correction of non-uniformities.